INSECTA 131 



serve to act and react throughout the body. The cells of the 

 metazoon are not merely related by nerves which bring them 

 under a central influence, but by a still more primitive means 

 of communication by chemical suggestions conveyed from one 

 organ to another, or to the. body as a whole, and so potent that, 

 rather than having been replaced by a nervous system, the 

 system has been perfected. This function is taken over by 

 the blood, which is the direct descendant of the mesenchyme. 

 The blood is not only a carrier of food, of oxygen, and of waste 

 material, it is the ever-circulating path along which chemical 

 impulses are conveyed. The impulses are related to environ- 

 ment, to growth, and to reproduction. The effects are made 

 known by definite movements resulting and by changes in 

 growth. Such impulses have proved so useful to the animal 

 that special organs have been developed to produce by secre- 

 tion the chemical agents which are distributed by the blood. 

 Such organs are called endocrine organs ; and the internal 

 secretions to which they give rise, endocrines or hormones. 

 But they are special manifestations of a system which concerns 

 and serves to relate the body generally. 



